Figure 1.

Pathway expression profiles could be distributed across cell types in different ways (schematic)

(A) Cell-cell signaling pathways comprise multiple variants of key components such as receptors (cartoons, R_n). These variants can be expressed in different combinations in different cell types. Colored dots identify receptor profiles for comparison with (B).

(B) Cell types can be arranged hierarchically based on similarities among their global (genome-wide) gene expression profiles (dendrogram). A hypothetical signaling pathway profile for each cell type is indicated by the gray intensity in the corresponding row of squares. In principle, each cell type could have a unique signaling pathway profile (unique, left); exhibit a smaller set of recurrent profiles, each used by a set of related cell types (recurrent and clustered, middle); or exhibit signaling pathway profiles that recur even among otherwise distantly related cell types (recurrent and dispersed, right). These possibilities are not exclusive, and it is possible that some pathways or subsets of cell types might operate in different regimes.